1. Technical Field
The present disclosure relates to thermoacoustic devices and speakers using the same, particularly, to a carbon nanotube based thermoacoustic device and a speaker using the same.
2. Description of Related Art
Speaker is an electro-acoustic transducer that converts electrical signals into sound. There are different types of speakers that can be categorized according by their working principles, such as electro-dynamic speakers, electromagnetic speakers, electrostatic speakers and piezoelectric speakers. However, the various types ultimately use mechanical vibration to produce sound waves, in other words they all achieve “electro-mechanical-acoustic” conversion. Among the various types, the electro-dynamic speakers are most widely used.
Referring to FIG. 43, the electro-dynamic speaker 300, according to the prior art, typically includes a voice coil 302, a magnet 304 and a cone 306. The voice coil 302 is an electrical conductor, and is placed in the magnetic field of the magnet 304. By applying an electrical current to the voice coil 302, a mechanical vibration of the cone 306 is produced due to the interaction between the electromagnetic field produced by the voice coil 302 and the magnetic field of the magnets 304, thus producing sound waves by kinetically pushing the air. However, the structure of the electric-powered loudspeaker 300 is dependent on magnetic fields and often weighty magnets.
Thermoacoustic effect is a conversion of heat to acoustic signals. The thermoacoustic effect is distinct from the mechanism of the conventional speaker, which the pressure waves are created by the mechanical movement of the diaphragm. When signals are inputted into a thermoacoustic element, heating is produced in the thermoacoustic element according to the variations of the signal and/or signal strength. Heat is propagated into surrounding medium. The heating of the medium causes thermal expansion and produces pressure waves in the surrounding medium, resulting in sound wave generation. Such an acoustic effect induced by temperature waves is commonly called “the thermoacoustic effect”.
A thermophone based on the thermoacoustic effect was created by H. D. Arnold and I. B. Crandall (H. D. Arnold and I. B. Crandall, “The thermophone as a precision source of sound”, Phys. Rev. 10, pp 22-38 (1917)). They used platinum strip with a thickness of 7×10−5 cm as a thermoacoustic element. The heat capacity per unit area of the platinum strip with the thickness of 7×10−5 cm is 2×10−4 J/cm2*K. However, the thermophone adopting the platinum strip, listened to the open air, sounds extremely weak because the heat capacity per unit area of the platinum strip is too high.
Carbon nanotubes (CNT) are a novel carbonaceous material having extremely small size and extremely large specific surface area. Carbon nanotubes have received a great deal of interest since the early 1990s, and have interesting and potentially useful electrical and mechanical properties, and have been widely used in a plurality of fields. Fan et al. discloses a thermoacoustic device with simpler structure and smaller size, working without the magnet in an article of “Flexible, Stretchable, Transparent Carbon Nanotube Thin Film Loudspeakers”, Fan et al., Nano Letters, Vol. 8 (12), 4539-4545 (2008). The thermoacoustic device includes a sound wave generator which is a carbon nanotube film. The carbon nanotube film used in the thermoacoustic device has a large specific surface area, and extremely small heat capacity per unit area that make the sound wave generator emit sound audible to humans. The sound has a wide frequency response range. Accordingly, the thermoacoustic device adopted the carbon nanotube film has a potential to be used in places of the loudspeakers of the prior art.
However, the carbon nanotube film used in the thermoacoustic device having a small thickness and a large area is easily damaged by the external forces applied thereon.